Effect of ventilation on acid-base balance and oxygenation in low blood- flow states

Objectives: To investigate how minute ventilation affects the partial pressure of end-tidal CO₂ and arterial and mixed venous pH, PCO₂, PO₂, and the concentration of bicarbonate during low blood-flow states. We tested the null hypothesis that acid-base conditions during low rates of blood flow are not significantly different when minute ventilation is doubled or halved. Design: Prospective, experimental, animal study. Setting: University hospital laboratory. Subjects: Domestic swine. Interventions: We studied ten anesthetized and mechanically ventilated swine (weight, 43 to 102 kg) in a new model of controlled systemic and pulmonary blood flow in which each animal was maintained on ventricular assist devices. After electrical induction of ventricular fibrillation, ventricular assist device blood flow was decreased in steps. At each decrease, control minute ventilation, two times the control minute ventilation (hyperventilation), and one-half the control minute ventilation (hypoventilation) were administered; each ventilatory change was maintained for 6 mins. Measurements and Main Results: Aortic, pulmonary arterial and central venous pressures, ventricular assist device blood flow, and end-tidal CO₂ were recorded continuously. Acid-base conditions were studied at three different mean blood flow rates: 49%, 30%, and 12% of baseline prearrest cardiac index. Arterial pH and PaO₂ and mixed venous pH varied directly (p < .003) with minute ventilation, while PaCO₂ and mixed venous PCO₂, and end-tidal CO₂ varied inversely (p < .0001) with minute ventilation. Mixed venous PO₂ was not significantly related to minute ventilation (p = .6). PaCO₂ and arterial bicarbonate; mixed venous pH, mixed venous PO₂, and mixed venous bicarbonate, and end-tidal CO₂ varied directly (p < .001) with blood flow, while mixed venous PCO₂ varied inversely with blood flow (p < .05). Arterial pH was not significantly related to blood flow (p = .3). When minute ventilation changed from hyperventilation to hypoventilation at a mean blood flow rate of 49%, mean arterial pH decreased 0.22 ± 0.06 (p < .05), mean PaCO₂ increased 28 ± 6 torr (3.7 ± 0.8 kPa) (p < .05), and mean PaO₂ decreased 99 ± 77 torr (13.2 ± 10 kPa); mean mixed venous pH decreased 0.11 ± 0.02, mean mixed venous PCO₂ increased 16 ± 2.2 torr (2.1 ± 0.3 kPa) (p < .05), and mean mixed venous PO₂ did not change; mean end-tidal CO₂ increased 18 ± 2 torr (2.4 ± 0.3 kPa) (p < .05). The effect of changes in minute ventilation on blood gases and end- tidal CO₂ was similar for mean blood flow rates of 30% and 12% of baseline cardiac index. Conclusions: During low rates of blood flow similar to those rates found in shock and cardiopulmonary resuscitation, alterations in minute ventilation significantly influenced end-tidal CO₂ and both arterial and mixed venous pH and PCO₂. These findings may have clinical importance in improving the treatment of shock and cardiac arrest.